In many industrial applications, formulated thermoplastic compositions require a balance of cost and performance. To that end, commodity materials such as polyethylene and polypropylene are attractive suitors from a cost standpoint, but are deficient in heat distortion temperature, modulus or tensile strength (herein referred to as mechanical properties). Classical efforts to improve the mechanical properties, such as glass reinforcement, will often result in deterioration of other beneficial properties. Additionally, known methods to improve the mechanical properties of thermoplastics often require substantial deviation in process operations and material requirements.
Polyolefins are useful in a wide variety of applications due to their intrinsic beneficial properties, including chemical stability, price point, and processability. Polyolefins, however, do not possess the thermal characteristics to compete against engineered thermoplastics. Some advances have been developed which overcome this deficiency to some degree, but generally the beneficial properties listed above are diminished. For example, the heat distortion temperature of polyolefins can be effectively improved by incorporation of high aspect ratio inorganic reinforcing additives, such as glass fibers, by crosslinking the substrate or by nucleation. Other desirable properties, however, are diminished by the inclusion of such additives.
Known methods for improving the mechanical properties of thermoplastics have focused on chemical modifications to the structure of the thermoplastic, changes to the crystallization characteristics of the thermoplastic, or crosslinking the thermoplastic composition, all of which require additional processes and equipment which can increase the operational costs for production of mechanically robust thermoplastics. For example, U.S. Pat. No. 4,990,554 describes a polyolefin composition comprising 75 to 97% polyolefin, and 25 to 3% by weight of a fibrous inorganic filler. This process theorizes that the underlining principle governing a material's heat distortion temperature is the ability to retain flexural modulus at elevated temperature. Consequently, high modulus inorganic fillers, such as glass or mineral, are added to the thermoplastic to impart an internal framework for resisting an applied load. The resulting compositions demonstrated an improvement in thermal deformation temperature, among other properties. This mechanism relies on the inorganic filler resisting conformal deformation in response to the applied load, or reducing the mobility of the polymeric chains at the organic-inorganic interface. Such reinforced thermoplastics may offer the desired mechanical properties, but they require additional materials which can increase the cost of production. As mentioned above, these methods also often reduce the native properties of the thermoplastics, such as flowability, processability, ease of conversion, and specific gravity.
U.S. Pat. No. 6,914,094 describes a polyolefin composition containing graft modified polyolefin-metal salt. The compositions were found to demonstrate improvements in both modulus and heat distortion temperature. The metal salt was introduced to the graft modified composition to neutralize the acid, and potentially form an ionomeric structure similar to a product sold by DuPont under the tradename Surlyn®. The metal salt is introduced to form an ionomeric structure that could impart inter- and intra-molecular forces to improve the mechanical properties of the thermoplastic. Accordingly, the improvement in the mechanical properties is derived by the presence of the ionomer. The inclusion of metal salts by this process results in a structural change in the modified thermoplastics, which is known to have an effect on the inherent properties associated with the host polymer. As a result, while certain mechanical properties are improved by this process, other desirable properties are sacrificed or lost.
A further known method to improve the mechanical properties of thermoplastics includes cross-linking. Crosslinked polyolefins are not a new topic of research, in fact there have been numerous investigations focusing on flame retardant compositions. In order to improve the thermal resistance, the polyolefin resin can be chemically crosslinked. For example, U.S. Pat. No. 5,378,539 describes a crosslinked composition that may contain an olefin resin that includes an olefin, metal hydroxide, coupling agents, a peroxide and a polyfunctional metal salt. The metal salt is believed to participate in the cross-linking reaction that is initiated by the presence of peroxide. The finished products have the improved flame retardant properties, and a balance of mechanical properties such as flowability and ease of conversion. However, the process is cumbersome and difficult to control in conventional compounding equipment. Additionally, the final composition is deemed a thermoset and is not reprocessible, a desirable characteristic of polyolefins.
Another method known in the art to improve mechanical properties in thermoplastics, such as polypropylene, is the use of metal salts as nucleation agents. For example, U.S. Pat. No. 6,645,290 describes a composition consisting of nucleation agents potentially comprising calcium, sodium or aluminum salts that impart improvements in crystallization kinetics. The compositions were found to have an improved flexural modulus, which is known to correlate to an improved heat distortion temperature. Nucleation is an effective tool for improving polypropylene properties, as it has an inherently slow crystallization rate. One deficiency of this technology is that is that is not effective with more rapid crystallizing polyolefins, such as the polyethylene family. Additionally, the use of metal salts as nucleators, and the resulting nucleation of the thermoplastic, requires additional materials which can increase the cost of production.